The correct answer is - introduce a price ceiling on drinking water.
With the introduction of price ceiling on drinking water the government will be able to properly regulate the water usage by putting on higher prices for it, but also to accommodate the prices according to the living standard to the population. By increasing the price the people will start using the water more rationally, they will stop using drinking water for watering the plants and for their yards, and also will be more careful to not leave the water running if there isn't need for that. It is always one of the most effective measures because it hits the consumers on their budget.